1. Field of the Invention
This invention relates to a light-emitting device having a light-emitting element for emitting light by fluorescence or phosphorescence. Particularly, this invention relates to a light-emitting device having an active element such as an insulated gate transistor or a thin film transistor, and a light-emitting element connected to the active element.
2. Description of the Related Art
A display device using liquid crystal, except for a reflection-type display device using external light, usually has a structure in which panels holding liquid crystal between them and a light source are combined to display an image. The liquid crystal display device is employed as image display means in various electronic devices but it has a disadvantage of a narrow view angle. On the other hand, a display device using a light-emitting material which provides electroluminescence has a wide view angle and excellent visibility. Therefore, this display device is noted as a next-generation display device.
In a light-emitting element utilizing electroluminescence, electrons injected from the cathode and positive holes injected from the anode are recombined to form excitons in a layer made of a light-emitting material (light-emitting layer) and energy emitted at the time when the ground state of the excitons is restored is taken out as light. Electroluminescence includes fluorescence and phosphorescence, which are understood as emission from a singlet state (fluorescence) and emission from a triplet state (phosphorescence) in the excited state, respectively. Since the luminance due to the emission reaches a range of several thousands to tens of thousands cd/m2, it is considered possible in principle to apply this electroluminescence to a display device or the like.
As an example in which a thin film transistor (hereinafter referred to as TFT) and a light-emitting element are combined, a structure in which an organic electroluminescence layer is formed via an insulating film made of silicon dioxide over a TFT using polycrystal silicon is disclosed. A passivation layer having a tapered edge part on the anode is situated below the organic electroluminescence layer. For the cathode, a material having a work function less than 4 eV is selected, and a magnesium alloy with a metal such as silver or aluminum is used (see Patent Literature 1).
It is known that organic compounds constituting the light-emitting element and alkaline metals or alkaline-earth metals used as electrodes react with water and oxygen and thus degrade. As a measure to prevent degradation due to moisture, a structure is known in which a plate-shaped protection casing for covering a display area is fixed with an adhesive or the like to a substrate over which the light-emitting element is formed while a desiccant is arranged on the inner side covered with the protection casing (see Patent Literature 2, for example).
Moreover, another structure is disclosed in which a display area is formed between a first substrate and a second substrate of different materials, with a planarization film formed as a buffer layer between a seal for adhering the first and second substrates and one of the substrates. Since the planarization film as a buffer layer is formed, the influence of thermal stress is reduced and separation of the seal and the substrate is thus prevented (see Patent Literature 3, for example).
The above-mentioned Patent Literatures are listed below.
Patent Literature 1: Japanese Publication of Laid-Open Patent Application No. H8-241047
Patent Literature 2: Japanese Publication of Laid-Open Patent Application No. H9-148066
Patent Literature 3: Japanese Publication of Laid-Open Patent Application No. 2001-102166
However, even though the substrate on which the light-emitting device is formed and the protection casing or the sealing substrate are hermetically sealed by the seal, degradation due to moisture such as a dark spot cannot be prevented. As the light-emitting element is electrified and driven, the current within the device is converted to Joule heat and the device is heated. In this case, a defect occurs such as a crack or breakage in the seal part or the coating at a bent part of the multilayer body due to a strain generated by the difference in thermal expansion coefficients between the constituent members, and it may be considered that a progressive defect such as a dark spot occurs from that part.
When forming a seal pattern for sealing around the display area formed from the light-emitting elements, in order to increase the adhesiveness and hermetic property of the seal are increased, the area required for the seal around the pixel region is increased, thus enlarging a so-called frame region. If such a panel is incorporated in a machine which requires a display panel, the size and design of the equipment are limited and its commercial value is lowered.